In the first half of the 20th century, the word Universe was used to mean the whole spacetime continuum in which we exist, together with all the energy and matter within it. Attempts to understand the Universe in this sense, on the largest possible scales, are made in cosmology, a science that has grown from physics and astronomy. During the second half of the 20th century, the development of observational cosmology, also called physical cosmology, led to a split in the meaning of the word Universe between observational cosmologists and theoretical cosmologists, where the former (usually) abandon the hope of observing the whole spacetime continuum, while the latter retain this hope, attempting to find the most reasonable speculations for modelling the whole of spacetime, despite the extreme difficulty in imagining any empirical constraints on these speculations and the risk of declining into metaphysics.
The terms known universe, observable universe, or visible universe are often used to describe the part of the Universe that we can see or otherwise observe. Those who believe it is impossible to observe the whole continuum may use our universe, referring only to that knowable by human beings in particular.
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The most important result of cosmology, that the Universe is expanding, is derived from redshift observations and quantified by Hubble's Law. Extrapolating this expansion back in time, one approaches a gravitational singularity, a rather abstract mathematical concept, which may or may not correspond to reality. This gives rise to the Big Bang theory, the dominant model in cosmology today. The time=zero of the Big Bang is estimated to have happened about 13.7 billion (13.7 × 109) years ago, with an uncertainty of only 200 million years, according to NASA's Wilkinson Microwave Anisotropy Probe.
A fundamental aspect of the Big Bang can be seen today in the observation that the farther away from us galaxies are, the faster they move away from us. It can also be seen in the microwave background radiation which is the much-attenuated radiation that originated soon after the Big Bang. This background radiation is remarkably uniform in all directions, which cosmologists have attempted to explain by an initial period of rapid inflation following the Big Bang.
It is not known whether the Universe is finite or infinite in spatial extent and volume, although the majority of theorists currently favor an finite Universe.
However, the observable universe, consisting of all locations that could have affected us since the Big Bang given the finite speed of light, is certainly finite. The edge of the cosmic light horizon is 14-15B light years distant. The present distance (comoving distance) to the edge of the observable universe is larger, since the universe has been expanding; it is estimated to be about 50 billion light years (4.7E23km). This would make the comoving volume, of the known universe, equal to 5E32 cubic lightyears (assuming this region is perfectly spherical). The observable universe contains about 7 × 1022 stars, organized in about 1010 galaxies, which themselves form clusters and superclusters. The number of galaxies may be even larger, based on the Hubble Deep Field observed with the Hubble Space Telescope.
The reader should be warned that both popular and professional research articles in cosmology often use the term "Universe" when they really mean "observable universe". This is because unobservable physical phenomena are philosophically irrelevant to human existence. Thus the term our...
We live in the centre of the observable universe, in apparent contradiction to the Copernican principle which says that the Universe is more or less uniform and it has no distinguished centre. This is simply because light does not travel infinitely fast, and we make observations of the past. As we look further and further away, we see things from epochs (times) closer and closer to the limit of time=zero of the Big bang model. And since light travels at the same speed in any direction towards us, we live at the centre of our observable universe.
An important open question of cosmology is the shape of the universe.
Firstly, whether or not the Universe is flat, i.e. whether the Pythagorean theorem for right triangles is valid on the largest scales. Currently, most cosmologists believe that the observable universe is (nearly) flat, just as the Earth is (nearly) flat.
Secondly, whether or not the Universe is multiply connected. The Universe has no spatial boundary (according to the standard Big bang model), but nevertheless may be spatially finite. This can be understood by a two-dimensional analogy: the Earth's surface has no edge, but nonetheless has a finite area. You can also think of a cylinder, and then let your imagination go free of ordinary space and imagine sticking the two ends of the cylinder together, but without bending the cylinder. This too is a two-dimensional space with a finite area, but, differently to the Earth's surface, it is flat, so it may serve as a better model.
Therefore, strictly speaking, we should call the above mentioned stars and galaxies "images" of stars and galaxies, since it is possible that the Universe is finite and so small that we can see once or several times around it, and the real number of physically distinct stars and galaxies could be a little smaller. There are observations underway to determine whether this is true.
Depending on the average density of matter and energy in the Universe, it will either keep on expanding forever or it will be gravitionally slowed and will eventually collapse back on itself in a "big crunch". Currently the evidence suggests not only that there is insufficient mass/energy to cause a recollapse, but that the expansion of the universe seems to be accelerating and will accelarate for the whole of eternity, see accelerating universe. For a more detailed discussion of other theories, see the ultimate fate of the Universe.
There is some speculation that multiple universes exist in a higher-level multiverse. For example matter that falls into a black hole in this universe could emerge as a big bang starting another universe. However all such ideas are currently not testable and so cannot be regarded as anything more than speculation.
Different words have been used throughout history to denote "all of space", including the equivalents in various languages of "heavens", "cosmos" and "world".
Although words like world and its equivalents in other language now almost always refer to the planet Earth, they used to refer to everything that exists - see for example Copernicus - and still sometimes do (as in "the whole wide world").
When speculating about a multiverse, one often thinks of it as consisting of many "universes" (lower case), our Universe being one of them.